In the field of semiconductor technology, research and development efforts are continued for further miniaturization of pattern features. Recently, as advances including miniaturization of circuit patterns, thinning of interconnect patterns and miniaturization of contact hole patterns for connection between cell-constituting layers are in progress to comply with higher integration density of LSIs, there is an increasing demand for the micropatterning technology. Accordingly, in conjunction with the technology for manufacturing photomasks used in the exposure step of the photolithographic microfabrication process, it is desired to have a technique of forming a more fine and accurate circuit pattern or mask pattern.
In general, reduction projection is employed when patterns are formed on semiconductor substrates by photolithography. Thus the size of pattern features formed on a photomask is about 4 times the size of pattern features formed on a semiconductor substrate. In the current photolithography technology, the size of circuit patterns printed is significantly smaller than the wavelength of light used for exposure. Therefore, if a photomask pattern is formed simply by multiplying the size of circuit pattern 4 times, the desired pattern is not transferred to a resist film on a semiconductor substrate due to optical interference and other effects during exposure.
Sometimes, optical interference and other effects during exposure are mitigated by forming the pattern on a photomask to a more complex shape than the actual circuit pattern. Such a complex pattern shape may be designed, for example, by incorporating optical proximity correction (OPC) into the actual circuit pattern. Also, attempts are made to apply the resolution enhancement technology (RET) such as modified illumination, immersion lithography or double exposure (or double patterning) lithography, to meet the demand for miniaturization and higher accuracy of patterns.
The phase shift method is used as one of the RET. The phase shift method is by forming a pattern of film capable of phase reversal of approximately 180 degrees on a photomask, such that contrast may be improved by utilizing optical interference. One of the photomasks adapted for the phase shift method is a halftone phase shift photomask. Typically, the halftone phase shift photomask includes a substrate of quartz or similar material which is transparent to exposure light, and a photomask pattern of halftone phase shift film formed on the substrate, capable of providing a phase shift of approximately 180 degrees between exposure light transmitted by a transparent section where no phase shift film is formed and exposure light transmitted by a (phase shift) section where the phase shift film is formed and having an insufficient transmittance to contribute to pattern formation. As the halftone phase shift photomask, there were proposed photomasks having a halftone phase shift film of molybdenum silicide oxide (MoSiO) or molybdenum silicide oxynitride (MoSiON) as disclosed in Patent Document 1, and photomasks having a halftone phase shift film of SiN or SiON.
In general, a photomask pattern is formed by furnishing a photomask blank having a light shielding film on a transparent substrate, coating a photoresist film on the blank, imagewise scanning the resist film with electron beam, developing the film to form a resist pattern, and etching the light shielding film into a light shielding pattern with the resist pattern used as an etching mask. It is desired to miniaturize the size of the light shielding pattern. If the resist film is processed while maintaining its thickness unchanged from that prior to miniaturization, then a ratio of film thickness to pattern size, known as aspect ratio, becomes high, and the shape of resist pattern features is degraded. This interferes with pattern transfer and sometimes causes the resist pattern to collapse or strip. Therefore, the thickness of resist film must be reduced to comply with miniaturization.
For the light shielding film which is etched using the resist pattern as an etching mask, a variety of materials have been proposed. In the commercial application, however, chromium compound films are used most often because of full knowledge of their etching behavior and their processing technique has been established as the standard. For example, light shielding films of chromium compounds are used in the photomask blank adapted for ArF excimer laser lithography, and specifically chromium compound films having a thickness of 50 to 77 nm are known from Patent Documents 2 to 4.
Typical of the dry etching to which chromium base films such as chromium compound films are subjected is oxygen-containing chlorine base dry etching, which often exerts an etching effect to organic films to a certain extent. When a chromium compound film is etched through a thin resist film pattern, accurate pattern transfer is difficult due to some etching of the resist film. It is a very difficult problem to require the resist to meet both a high resolution and sufficient etch resistance to enable a high accuracy of etching.
On the other hand, the attempt to use a hard mask in order to reduce the load on the resist during dry etching is common in the art. For example, Patent Document 5 describes that a SiO2 film is formed on a MoSi2 layer and used as an etching mask when MoSi2 is dry etched with chlorine-containing gas. The SiO2 film also functions as an antireflective film.